Review of the 1755 Lisbon Earthquake Based on Recent Analyses of Historical Observations - Historical Seismology: Interdisciplinary Studies of Past and Recent Earthquakes - page 266

The influence of the topography, of the morphology of the geological layers, the
influence of three-dimensional deep geology, of the effect of important discontinu-
ities inside the geological structures, of the three-dimensional aspects of the wave
propagation, etc, are some other very important aspects that have been relegated to
a second level until now. Only recently, new projects are addressing topics related
to these aspects.
This theme, which is very important to determine with accuracy the effects of
future earthquakes in the city of Lisbon, needs a conjugation of efforts of different
areas of knowledge, including geology, seismology, geotechnical and earthquake
engineering, along with the effort of analytic modulation and calibration through a
careful monitoring as complete as possible. Oliveira (2004) makes the confrontation
of the developed models for the city of Lisbon, using different scales of work and
concluding that, although the general results are similar, the differences can be really
large when seen in detail.

3.2 General Characteristics of the Earthquake

3.2.1 The Seismic Source

Based on the similarity of isoseismals and on the connection done during a long time
between this earthquake and the one of February 28, 1969, the epicentral location
of 1755 was placed in the region of the epicentre of 1969. Former studies placed
the epicentre in different places of the Atlantic, more or less near Lisbon. Although
the much research done in the last 10 years supported by studies which will be
referred ahead, mainly the tsunami, the information about the crust properties, and
the engineered behaviour of different structures, several source mechanisms are still
disputing the origins of the 1755 earthquake (Fig. 6):

- A wide geological fault structure split in two large lengthy areas that develop in

parallel to the west coast and south of Mainland Portugal (Ribeiro 2002, Zitellini
et al. 1999).

- A phenomenon build up by two main episodes, one originated in one of the

sources already described, and another located somewhere in the region of the
inferior Tagus Valley (Vilanova et al. 2003).

- A phenomenon originated by the interaction of the Alboran Plate with the Euro-

Asian Plate in a region of incipient subduction (Gutscher 2004).

- A simple model of rupture at deeper location at a “sub-horizontal” dislocation

area.

These models require the use of more complex wave propagation schemes, intro-
ducing fault directivity, anisotropy elastic medium and three-dimensional (lateral
heterogeneous) modelling.
The arguments put forward by the different teams working in this theme are
several, and they do not take enough evidence to allow us to choose one among the